HomeFix: Water-heater woes stem from pressure imbalance

Wednesday

Aug 24, 2011 at 12:01 AMAug 24, 2011 at 11:38 AM

Q: They worked properly until about a month ago when my carbon-monoxide detector sounded. It was determined that the problem was the exhaust from the gas water heater. The water-heater flue was found to be clear but a bit short, so we added to it. The water heater vented properly only when the air conditioner wasn't running.

Dwight Barnett

Q: They worked properly until about a month ago when my carbon-monoxide detector sounded. It was determined that the problem was the exhaust from the gas water heater. The water-heater flue was found to be clear but a bit short, so we added to it. The water heater vented properly only when the air conditioner wasn't running.

Over the next two weeks we did several things to attempt to correct the problem, none of which did. The ducts under the house were old and leaking air, so they were replaced. We checked the air return, and it appeared to be drawing well. We put new filters in the furnace. We sealed the register in the garage. None of this corrected the problem.

Finally, the air-conditioner man cut a little flat round disk out of the top of the water heater. This did appear to allow the water-heater exhaust to vent correctly. I don't trust it, so I only turn the water heater on when I turn the air conditioner off. The AC is still pulling air through the clothes dryer and everywhere else possible. What would correct the under-pressure problem?

A: When the air conditioner is on, the furnace fan is pushing air through the supply ducts and pulling air through the return ducts. The newer fan system is circulating more air than the older unit, and the pressures inside the home have changed. If the return ducts are not sealed at all joints and seams, they will pull air from the crawlspace, attic, the water-heater flue and dryer vent. You also need to make sure the cold-air return system is balanced with the supply system because the whole house is sucking air.

What you have is a negative-pressure problem in the combustion air zone, which is generally a small utility room where all the combustion appliances are located. A simple solution is to keep the door to the furnace room open or add a louvered door to provide make-up air to the fan system.

If the open-door policy doesn't work, the problem areas can be found by using a blower door test to simulate the negative pressures inside the home. While the blower-door fan is running, a trained energy auditor will check areas of the home that do not have sufficient air returns to supply the furnace fan.

In extreme cases, it might be necessary to bring in an outside fresh-air supply for the home's combustion appliances, from either the crawlspace or the attic. Whichever you choose, the pipe should be covered with a screen to prevent pest entry. The pipe should extend up or down to be near the burners on the water heater.

If the problems persist, I would recommend you install a sealed combustion water heater, or a high-efficiency power-ventilated water heater. Either one would save energy and prevent back-drafting of flue gases.

Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home-improvement questions at C. Dwight Barnett, Evansville Courier & Press, P.O. Box 268, Evansville, Ind. 47702 or e-mail him at d.Barnett@insightbb.com.

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